The present invention relates to soft-shutdown circuits and, more particularly, to soft-shutdown circuits used in gate drive circuits to provide over-current protection of loads, such as motors.
In motor drive applications, over-current protection is often required in order to protect the switching power devices employed from catastrophic destruction due to excess heat concentration of the devices. Complete protection circuit normally consists of sensing an over-current condition, reporting of the condition, and shutting down the gate drive circuit.
A soft-shutdown method is often preferred over a hard-shutdown method. This is primarily due to the fact that soft-shutdown results in less overshoot voltage across the switching devices since it limits the rate of change of short circuit current (di/dt). Even if the circuit has a parasitic inductance in the series path of the short-circuit current, this controlled di/dt will substantially reduce excess overshoot voltage across the switching devices.
However, during the soft-shutdown period, all other gate signals need to be unchanged in order to complete the soft-shutdown sequence. Otherwise, if any of the other gate signals, which carry large over-current in associated switching devices, turn off while soft-shutdown is in process, the soft-shutdown is aborted by a hard-shutoff of the other devices.
If the gate drive circuit uses multiple integrated circuits or discrete circuits to drive all of the power devices for AC motor drive application, typically six devices, implementation of a soft-shutdown scheme has proven to be difficult since each gate drive circuit needs to be synchronized with the others to ensure successful completion of the soft-shutdown process.